1. Field of the Invention
The present invention relates in general to a high sensitivity complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same. More particularly, the present invention relates to a CMOS image sensor having excellent photosensitivity and picture quality by gathering a greater amount of light incident outside the sensor to thereby improve its peripheral brightness ratio.
2. Description of the Related Art
In general, an image sensor is a semiconductor device which converts an optical image to an electric signal. There are two types of image sensors: charge coupled devices (CCDs) and complementary metal-oxide semiconductors (CMOS) image sensors.
Particularly, a CCD is a device having closely arranged metal-oxide-silicon (hereinafter referred to as ‘MOS’) capacitors in which charge carriers are stored respective MOS capacitors and transferred. A CMOS image sensor utilizes CMOS technology employing a control circuit and a signal processing circuit as peripheral circuits for forming as many MOS transistors as the sum of existing pixel numbers in the peripheral circuit. In using these MOS transistors, the CMOS image sensor adopts a switching mode which sequentially senses outputs.
So far, the CCD has been the image sensor receiving the most attention, and it is still used broadly in many applications including digital cameras and camera phones. However, with recent rapid popularization of camera phones, there is a need for low-power consumption. To keep abreast of this trend, investigators have now turned their attention to the CMOS image sensor. This is because the CMOS image sensor can be easily produced using a general CMOS process for fabricating silicon semiconductors, has a small in size and is cost effective, and has low power consumption. Even though it is apparent that the CMOS image sensor is excellent as a portable sensor because of its high degree of integration and low power consumption, the CMOS image sensor has very low photosensitivity compared to a related art CCD. There have been a number of studies which seek to overcome this problem. Meanwhile, another recent tendency is to reduce size of mobile equipment including camera phones. Accordingly, an image optical system housed in a camera phone needs to have a compact size and high picture quality. For example, as the total number of pixels of the CMOS used in a camera phone increases to 300,000, 1 million, 1.3 million, 2 million, 3 million and so forth, the pixel size of a sensor must gradually decrease. Also, the diagonal length of the image sensor must be short if the image module is to have a compact size. Along with these trends, the back focal length is also being made shorter. Because of this, the incident angle of light passing through an edge of the CMOS is getting larger. In other words, the brightness ratio of light incident on the central part to the edge of the CMOS (hereinafter, referred to as a “peripheral brightness ratio”) is reduced in proportion to the back focal length of a lens. This phenomenon is observed not only in the CMOS but also in the CCD. In effect, it is one of the largest concerns related to sensors that needs to be resolved.
FIG. 1 is a schematic diagram of an image sensor having an improved photosensitivity disclosed in U.S. Pat. No. 4,667,092. Referring to FIG. 1, to improve the photosensitivity in a related art CCD image sensor, micro lenses ML are deposited on the upper portion of photodiodes PD. In general, the photodiode PD occupies only a certain part of the pixel area of the image sensor. Therefore, the fill factor occupied by the photodiode in the pixel area is less than 1, and accordingly, part of the incident light is inevitably lost. Micro lenses ML are disposed on the upper portion of the photodiodes PD to condense the lost incident light and thus, to increase the quantity of light focused on the photodiodes PD.
FIG. 2 comparatively illustrates a light source that is incident perpendicularly to the sensor of FIG. 1 (FIG. 2A), and an incident light source forming an inclined angle with the sensor of FIG. 1 (FIG. 2B). When the focus position of the micro-lens ML is formed on the photodiode PD, the light incident perpendicularly to the micro-lens ML fully converges on the photodiode PD. However, if the incident light is tilted so that it strikes the micro-lens ML at an angle, a certain deviation length occurs. This phenomenon is observed in light beams incident upon the center and peripheral sides of the sensor, and is a main cause of deterioration of the peripheral brightness ratio of the image sensor. The best-known answer so far to solve the peripheral brightness ratio problem is to reduce the size of the micro-lens.
FIG. 3 is a schematic diagram of an image sensor disclosed in U.S. Pat. No. 5,601,390. Particularly, FIG. 3A illustrates a state in which the optical axes of the micro lenses ML and the optical axes of the photodiodes PD are not coincident, wherein a mask for use in fabricating the micro-lens ML is reduced by a certain ratio. FIG. 3B illustrates a state in which light is incident at a certain angle because of the non-coincident optical axes of the micro-lens ML and the photodiode PD, where the incident light converges on a photodiode PD. In the case of the image sensor illustrated in FIG. 3, the optical axes of the micro-lens ML and the photodiode PD become more distant from each other at the periphery of the sensor than at its center. In this manner, more light can be gathered that is incident at the periphery of the sensor, and the peripheral brightness ratio can be improved. However, there is a fatal flaw in this method. Since the angle of incidence of light entering the center of the sensor is different from the angle of incidence of light entering the periphery of the sensor, if the lenses have the same physical properties, e.g., focal length and lens' diameter, a sufficient quantity of light cannot converge on the photodiode. To solve this problem, in other words, to improve uniformity of light converging on the photodiode (or the peripheral brightness ratio), the micro lenses disposed at the center and the peripheral part of the sensor are designed to have different physical properties. Unfortunately though, a related art fabrication method for the micro-lens, wherein a photoresist (PR) is patterned in rectangular or cylindrical shape and heated to form a micro-lens, cannot meet this requirement.
FIG. 4 is a schematic diagram of an image sensor employing an inner layer lens. As shown in FIG. 4, a light beam that is once converged by a micro-lens ML is converged again by an inner layer lens disposed in the vicinity of a photodiode PD. In this manner, the light gathering efficiency is increased. Particularly, the use of the inner layer lens is very effective for improving the peripheral brightness ratio in that it converges not only a light beam that is incident perpendicularly to the sensor, but also a light beam that is incident at an angle. Because of this merit, an inner layer lens is already employed in many CCD image sensors. Unlike the CCD, however, due to its structure, it is difficult to accommodate an inner layer lens in the CMOS. Even if successful, two or three additional masking steps are required, which increases the cost of manufacturing the CMOS.
FIG. 5 diagrammatically illustrates how the distance between a micro-lens and a photodiode affects the quantity of light incident on the micro-lens at an angle that is gathered by the image sensor. Particularly, FIG. 5A illustrates a case in which the distance H1 between the micro-lens ML and the photodiode PD is relatively long; and FIG. 5B illustrates a case in which the distance H2 between the micro-lens ML and the photodiode PD is relatively short. According to the nature of a lens, if the focal length is long, the focal deviation W1 is increased, while if the focal length is short, the focal deviation W2 is decreased. Because of these characteristics, if the focus of the micro-lens is on a photodiode, the distance between the micro-lens and the photodiode should not be too long because the quantity of light converging on the photodiode is inversely proportional to the distance. Referring to FIG. 5C showing the distance H between the micro-lens ML and the photodiode PD, light beams incident perpendicularly to the micro-lens all converge on the photodiode. On the other hand, light beams incident on the micro-lens at an angle are reflected by a peripheral structure of the photodiode. This phenomenon occurs more often when the distance between the micro-lens and the photodiode is increased, resulting in an increase in focal deviation. Therefore, to improve the peripheral brightness ratio, it is important to minimize the distance between the micro-lens and the photodiode. However, unlike the CCD, structural problems in the CMOS interfere in reducing the distance between the micro-lens and the photodiode.